NO. 6 CHIGGER SUBGENUS GAHRLIEPIA — TRAUB AND MORROW 83 



quire heavy infestations of such a species of chigger. Physiological 

 host preferences, in contrast, refer to regular or fairly specific infes- 

 tations of a particular species of host, regardless of the ecological 

 habitat involved. The instance of Trombicula (Leptotrombidium) 

 pallida (Nagayo et al., 1919) in Korea may be cited as an example 

 of ecological preference. Here it was found that 61 percent of the 

 chiggers from the reed vole, Microtus fortis pelliceiis Thomas, were 

 T. pallida (Traub et al., 1953). Such a high percentage at first 

 glance suggests a large degree of host specificity. However, 40 per- 

 cent of the chiggers on the black-striped field mouse, Apodemus 

 agrarius mantchuricus Thomas, also were T. pallida, and, in fact, 67 

 percent of all the T. pallida examined were from A. agrarius. In con- 

 trast, less than i percent of the chiggers on the red-backed vole, 

 Clethrionomys rufucanus regidus Thomas, were T. pallida. Microtus 

 could be found only in very specialized habitats — the thick grass 

 growing along the side of a few streams. On the other hand, Apode- 

 mus agrarius was common everywhere. Along streamsides it was 

 just as abundant as Microtus, and it frequently was found in such 

 habitats when Microtus was apparently absent. Clethrionomys in- 

 habited the woody slopes 150 feet away from such streamsides and 

 never was trapped near the water. T. pallida was therefore appar- 

 ently most prevalent along the margins of streams, and parasitized 

 the mice that inhabited such areas rather than favoring Microtus 

 for purely physiological reasons. Unfortunately, the precise ecologi- 

 cal requirements are as yet largely unknown for most groups of 

 trombiculids, but it has been shown that the degree of ground mois- 

 ture is a major factor in determining which of two North American 

 species, Tromhicida (Eutrombicula) alfreddiigesi (Oudemans) or 

 T. (E.) splendens (Ewing), may be found in circumscribed areas, 

 and the two populations may coexist only 100 feet apart (Wharton 

 and Fuller, 1952). 



The subgenus Gahrliepia, as a whole, exhibits some rather pro- 

 nounced host preferences, which to a great extent are apparently 

 physiological and not ecological. That is, they are apt to be found 

 regularly or frequently on certain kinds of mammals but not on others 

 found in the same environment and with much the same habits, as 

 illustrated in North Borneo on Mount Kinabalu at Tenompak (4,500 

 feet elevation). Although Rattus w. whiteheadi Thomas, Rattus 

 cremoriv enter Miller, Rattus a. alticola Thomas, and Rattus rajah 

 Thomas were frequently trapped alongside the lesser gymnuran, 

 Hylomys suillus dorsalis Thomas (an insectivore), these mammals 

 differed greatly as regards their infestations of chiggers. Hylomys 



